Credit card readers have been devised which rely on magnetic codes stored on the card in the form of binary data. A simple and reliable code scheme is a bar code known as the "Aiken code." In this scheme binary data is stored on the magnetic tape in the form of magnetic domain transistors spaced along the tape. The presence or absence of a transistor (actually an interface between oppositely poled magnetic domains) indicates a binary one or zero. Since this data is represented specifically along the tape axis the reading speed must be carefully controlled at a uniform rate to coincide with a clocking arrangement incorporated into the reader. This requires typically a motorized arrangement to feed the card passed the sensing head of the reader.
Systems are available in which timing data is incorporated into the magnetic tape so the card reader can sense the instantaneous rate of tape travel and adjust accordingly the clocking circuit in the reader. By comparison with the system described here, such systems are unnecessarily complex and have difficulty in responding if the card reading rate is interrupted or grossly varied.
A simple credit card reader that overcomes the foregoing, at least in part, is described and claimed in application Ser. No. 553,528 filed Feb. 27, 1975 in the name of G. E. Moore, Jr. In this device two sensing elements are spaced one-half cell length apart to provide in response to the code information independent outputs which when applied to simple logic circuitry provide separate representations of the data and the timing information. The logic circuitry includes decisional circuitry responsive to outputs from the two elements for generating an output stream containing timing information. The timing information in conjunction with the outputs of both of the sensor elements is used to generate a separate output stream containing data.
In a preferred embodiment, a pair of magnetoresistive sensor elements are spaced one-half cell length apart in the path of the code. Each element responds independently to the regular transitions at the beginning and at the end of a cell and also to the irregular transitions (data information) stored in the center of the cell. Due to the spacing between elements, an output is received from the two elements simultaneously only when a data bit (a binary one) is stored. Only one element is activated when no data bit (a binary zero) is stored. The electronic output is independent of the speed at which the code passes the sensor.
A simple electronic circuit processes the outputs of the two elements typically for transmission to a remote computer. The outputs are applied to an OR circuit, the output of which is employed both to enable the data from the elements to be applied (via an AND gate) to a shift register and to provide a clock pulse for the shift register. The operation provides enabling pulses with widths reflective of the movement of the credit card and the effect is that the output from at least one of the elements is stored in the shift register whenever the corresponding bits of the code arrive at the sensor.
The operation depends on the close proximity of the elements to one another. In order to achieve the desired proximity, magnetoresistive sensor elements are formed by photolithographic techniques and are connected electrically in parallel to provide independent indications of the code as required. The constraint on the magnetoresistive elements impose design criteria realized in a novel approach herein. Each element, for example, includes a plurality of subelements having a prescribed growth or shape anisotropy. The subelements are connected electrically in series and respond to the presence of a transition of the code by the rotation of the magnetization therein.
From the foregoing it is evident that in this equipment the magnetic sensing head is an important element and it is vital that the integrity of the magnetic sensing elements and the space between them be precisely preserved. This requires a protective layer into which the sensing elements are embedded. The layer should be of a material that is nonmagnetic (so as not to interfere with the magnetic data being sensed) durable and in which the sensing elements can be rigidly suspended by a simple casting or potting process.
Various kinds of protective coatings for magnetic sensing heads have been proposed in the art. Well recognized are the requirements for hardness and durability, for the sensor should withstand exposure to a tape which contains hard particles of magnetic oxide that can be severely abrasive over a period of continued use.
U.S. Pat. No. 3,249,700 issued May 3, 1966 to S. Duinker et al proposes a glass protective layer. Glasses, aside from their tendency to crack and devitrify, do not possess outstanding wear resistance.
U.S. Pat. No. 3,417,386 issued Dec. 17, 1968 to R. A. Schneider, suggests a metal or metal alloy coating or a carbide coating. However, metal and alloy coatings, some of which are known for hardness, are not very wear resistant. Carbides tend to spall and gall. When the latter occurs the magnetic tape of the credit card processed through the machine is often damaged. Materials allegedly superior to these are described in U.S. Pat. No. 3,665,436 issued May 23, 1972 to J. J. Murray, et al. They purpose ceramic layers such as chromium oxide. Such materials, typically applied by plasma plating, have been used in the industry with some success.
However, we have now discovered a protective coating for use with credit card readers, and advantageously with the reader described herein, that exhibits improved durability, can be applied with a solvent free coating process (in the spirit of OSHA standards), can be patterned by photodeposition or screen printing prior to curing to final hardness, and is inexpensive. The coating is a polymer of one or more of the following monomers:
(a) 1-butane carbamic acid 2-methacryloyloxyethyl ester, PA0 (b) 1-butane carbamic acid 2-acryloyloxyethyl ester, PA0 (c) a mixture of isomers 2,2,4 - and 2,4,4,- trimethyl-1, 6-hexane dicarbamic acid di(2-methacryloyloxyethyl)ester, PA0 (d) a mixture of isomer as above in (c) in which the esters are both 2-acryloyloxyethyl, PA0 (e) di(4-cyclohexylcarbamic acid 2methacryloyloxyethyl ester)methane, PA0 (f) the same as in (e) in which the esters are 2-acryloyloxyethyl.
The monomers may be prepared by first reacting a hydroxy substituted ester of acrylic or methacrylic acid with a mono- or difunctional isocyanate. The reaction of the hydroxyl group with the isocyanate yields a urethane. The reaction product can be polymerized readily by heat, light or electron beams. Photo-initiated polymerization implies that the coatings can be patterned by standard lithography. The polymerization advantageously involves a high degree of cross-linking of the monomers until the system becomes essentially saturated (above 70%). This class of materials and techniques for their preparation are described in U.S. Pat. No. 3,479,328 patented Nov. 18, 1969.
In a preferred form of the invention the coating comprises a polymerized mixture of mono- and difunctional monomers advantageously in a weight ratio of mono- to difunctional monomer of 9:1 to 4:6. Thus with the monomers a-f, a or b could be mixed with any of c through f. Normally the acrylates would be mixed and the methacrylates mixed so that the most typical examples would be: a with c or e and b with d or f. Ternary mixtures are also useful.
One of the properties of interest is the toughness of the polymer. This characteristic is a measure of the amount of energy the material can absorb without losing structural integrity. The latter limitation is referred to in terms of the yield point and the break point. The preferred range of weight ratios of the mono- and difunctional monomers have to do with this characteristic of the resulting polymer. If the weight ratio exceeds 9:1 the yield point of the polymer is lower than desired although, with certain formulations and for some applications, mixtures that exceed that ratio may still be found useful. As the ratio falls below 4:6 the resulting polymer becomes hard and brittle, and breaks before reaching a yield point. However again, special cases may allow a departure from this preferred limit.